1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure and cutting element for such bits.
2. Background Information
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip”of the drill string, requires considerable time, effort and expense. Because drilling costs are typically thousands of dollars per hour, it is thus always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness. The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration “ROP”), as well as its durability or ability to maintain an acceptable ROP.
One common earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the bit is rotated, thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones, cone cutters, or the like. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones removes chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill string and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutter elements on the rotating cone cutters break up the formation to form a new borehole by a combination of gouging and scraping or chipping and crushing. The shape and positioning of the cutter elements (both steel teeth and tungsten carbide inserts) upon the cone cutters greatly impact bit durability and ROP and thus, are important to the success of a particular bit design.
The inserts in TCI bits are typically positioned in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates. Conventional bits also typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. Still further, conventional bits typically include a number of inner rows of cutter elements that are located in circumferential rows disposed radially inward or in board from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row or bottomhole cutter elements.
Inner row inserts in TCI bits have been provided with various geometries. One insert typically employed in an inner row may generally be described as a “conical” insert one having a cutting surface that tapers from a cylindrical base to a generally rounded or spherical apex. Such an insert is shown, for example, in FIGS. 4A-C in U.S. Pat. No. 6,241,034. Another common shape for an insert for use in inner rows is what generally may be described as a “chisel” shaped insert. Rather then having the spherical apex of the conical insert, a chisel insert generally includes two generally flattened sides or flanks that converge and terminate in an elongate crest at the terminal end of the insert. The chisel element may have rather sharp transitions where the flanks intersect the more rounded portions of the cutting surface, as shown, for example, in FIGS. 1-8 in U.S. Pat. No. 5,172,779. In other designs, the surfaces of the chisel insert may be contoured or blended so as to eliminate sharp transitions and to present a more rounded cutting surface, such as shown in FIGS. 3A-D in U.S. Pat. No. 6,241,034 and FIGS. 9-12 in U.S. Pat. No. 5,172,779. In general, it has been understood that, as compared to a conical inset, the chisel shaped insert provides a more aggressive cutting structure that removes formation material at a faster rate for as long as the cutting structure remains intact. For this reason, in soft formations, chisel shaped inserts are frequently preferred for bottom hole cutting.
Despite this known advantage of chisel shaped inserts, however, such cutter elements have shortcomings when it comes to drilling in harder formations, where the relatively sharp cutting edges and ends of the chisel endure high stresses that may lead to chipping and ultimately breakage of the insert. Likewise, in hard and abrasive formations, the chisel crest may wear dramatically. Both wear and breakage may cause a bit's ROP to drop dramatically, as for example, from 80 feet per hour to less than 10 feet per hour. Once the cutting structure is damaged and the rate of penetration is reduced to an unacceptable rate, the drill string must be removed in order to replace the drill bit. As mentioned, this “trip” of the drill string is extremely time consuming and expensive to the driller.
Another known phenomena detrimental to drill bit life and ROP is a abrasive wear that tends to wear away and flatten the cutter element on the side generally facing the borehole wall. As this wear occurs, the cutter element removes less formation material with each strike of the insert against the formation, typically leading to reduced ROP. In addition, wear may result in greater side wall forces imparted on the bit. Such increased loads tend to place greater demands and stresses on the bearings and may lead to bit instability and wobble which, in turn, may cause the bit to deviate from its intended drilling path. Further, as the surface of the insert facing the borehole wall tends to wear toward the center of the chisel structure (i.e., the chisel structure wears from the outer edge towards the center), the insert becomes sharper, and more likely to chip and ultimately to break.
Accordingly, there remains a need in the art for a drill bit and cutting elements that will provide a relatively high rate of penetration and footage drilled, yet be durable enough to withstand hard and abrasive formations. Such drill bits and cutting elements would be particularly well received if they had geometries adapted to resist such off center wear, and further, when such wear nevertheless does occur, to resist the tendency for the cutter element to break.